The present invention relates to navigation satellite receivers, and more particularly to methods and systems for improving the time-to-first-fix of navigation satellite receivers with assistance being provided by a network server.
Global positioning system (GPS) receivers use signals received from several earth-orbiting satellites in a constellation to determine user position and velocity, and other navigational data. A navigation receiver that has just been turned on does not yet know where it is, how much its crystal oscillator is in error, nor what time it is. All these are needed to find and lock onto the satellite transmissions, and so a search must be made of all the possibilities.
Each GPS satellite vehicle (SV) transmits navigation (NAV) data at fifty bits-per-second, and includes ephemeris, clock and almanac information. Such information allows a GPS receiver to compute its position, velocity, and time. One whole data frame of NAV data is 1500-bits long, and thus takes thirty seconds to transmit.
Each data frame is divided into five subframes 1-5, and each subframe is 300-bits long, e.g., ten 30-bit words. Thus it takes six seconds to transmit each 300-bit, 10-word subframe. Every subframe starts with a telemetry (TLM) word of 30-bits, followed by a hand-over word (HOW) of 30-bits. Both 30-bit words comprise 24-bits of data and 6-bits of parity. There are eight words of data payload in each subframe.
The TLM word at the front of each 300-bit subframe begins with an 8-bit preamble. The preamble allows the start of a subframe to be recognized, and thereafter provides a primary mechanism for the receiver to be synchronized.
The first 300-bit subframe transmits the SV-clock correction data after the TLM word and HOW. The second subframe transmits the first part of the SV-ephemeris data. The third subframe transmits the second part of the SV-ephemeris data. Subframes four and five are used to transmit different pages of system data. The fourth subframe also begins with the TLM word and HOW, and the data payloads rotate over 12.5 minutes to transmit the lengthy information about the ionosphere, UTC, and other data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over such 12.5 minute period. The fifth subframe begins with the TLM word and HOW, and its data payload also rotates over 12.5 minutes to transmit the rather large almanac.
The clock data parameters describe the SV clock and its relationship to GPS time. The ephemeris data parameters describe SV orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set. The almanacs are approximate orbital data parameters for all SV""s. The ten-parameter almanacs describe SV orbits over extended periods of time, and is sometimes useful for months.
The signal-acquisition time of a GPS receiver at start-up can be significantly speeded by having the current almanac available. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency of each SV in the constellation.
So in prior art devices, the synchronization of the receiver to NAV data transmissions depends on the detection of the preamble pattern (xe2x80x9c8Bxe2x80x9d) in the TLM word. Since the TLM word is only transmitted every six seconds, it can take three seconds or more to detect it. Such delays degrade time-to-first-fix (TTFF) performance, an important user criteria.
It is therefore an object of the present invention to provide a method and system for navigation satellite reception and receiver initialization that can proceed without actually receiving the preamble in the TLM word.
It is another object of the present invention to provide a method and system for shortening the time needed for initialization of navigation devices.
It is a further object of the present invention to provide a satellite-navigation system that is inexpensive.
Briefly, a navigation-satellite receiver embodiment of the present invention uses a reference station to store NAV data subframes for pattern matching at a client. Alternatively, the pattern matching is performed by the server when per-byte communication costs are high. The stored NAV repeats ephemeris data every thirty seconds, and full almanac data every 12.5 minutes. This permits the client to quickly recognize where in the NAV data sequence its own received signals are, and need not actually wait to receive the preambles in the TLM words. Several precious seconds are therefore saved in producing a rapid time-to-first-fix.
An advantage of the present invention is that a system and method are provided that produce faster initialization times in navigation satellite receivers.